This invention generally relates to medical apparatus for measuring characteristics of a heart. More particularly, the invention relates to a balloon flotation electrode catheter which can be used with appropriate equipment to monitor cardiac outputs on a beat-by-beat basis over a prolonged period of time.
While the invention is particularly applicable to the measurement of cardiac output in the right ventricular chamber of a human heart, it should be appreciated that the measurement of cardiac output in another chamber of a heart, such as the left ventricular chamber and of a nonhuman heart such as a suitable mammalian heart can also be performed by the present invention.
Several parameters are routinely monitored in patients having heart problems or those undergoing cardiovascular surgery. These include the electrocardiogram (ECG), the arterial blood pressure (ART), the central venuous pressure (CVP), the pulmonary artery pressure (PAP), and the cardiac output (CO). With the exception of cardiac output, technology now exists which permits these time varying parameters to be monitored continuously. However, all present techniques for clinically obtaining cardiac output involve indirect methods with sample intervals of several minutes. In addition, these techniques require either the injection of an indicator substance or the gathering of significant respiratory and blood gas patient data.
Cardiac output is generally measured in terms of liters per minute which corresponds to the heart's stroke volume multiplied by heart rate. Cardiac output values change depending on the activity level of the body, the level of metabolic demand, the condition of the heart and many other factors. During major operations, cardiac output is clinically significant because it is an indicator of how well the heart itself is performing and it demonstrates whether a sufficient supply of blood is being circulated to maintain metabolic demands.
One of the indirect methods of measuring cardiac output is the Fick method which determines such output by examining both the oxygen consumption of the lungs and the difference between arterial and venuous oxygen concentrations. A second method involves indicator dilution. Early indicator techniques used injectates such as cardio green dye which was injected as a bolus into the vascular system and allowed to mix with the venuous blood. An arterial sampling through a densitometer was then used to measure the time varying concentration levels of dye. The concentrations recorded were directly related to the flow rate of the dye mixed blood through the circulatory system.
The currently accepted clinical indicator method is a technique known as thermodilution. This method relies on thermal changes as a flow indicator. A bolus of cold fluid, at least 10.degree. C. less than the patient's core temperature, is injected into a venuous site. After mixing in the right ventricle, the adjacent cooled blood and fluid pass a small thermistor temperature sensor which has been placed via a catheter in the patient's pulmonary artery. The time varying temperature changes are directly related to the flow rate of the mixed fluid through the right side of the heart. Since the circulatory system is a series circuit, the right side value is also reflective of the left side ejections. Thus, a cardiac output can be calculated from the indicator dilution curve using a known equation.
Non-invasive techniques for obtaining cardiac output have been recently developed. Echocardiographic instruments can be used to measure aortic sizes and ventricular volumes at specific times during the cardiac cycle. Stroke volumes can then be derived from this information. In this connection, flow doppler instruments have been developed to measure blood velocity via external probes which are placed on the skin of the patient and aimed at a major arterial vessel such as the ascending or descending aorta. Cardiac output is then derived by estimating the vessel diameter in determining blood flow. Further calculations can convert the flow determinations to cardiac output by multiplying the heart rate and the flow per beat. Also, instruments which attempt to measure transthoracic impedance have also been developed in an attempt to determine non-invasive cardiac output. Finally, a non-invasive technique known as the pulse wave contour technique has been developed which makes use of the concept that the area under the arterial waveform curve is related to the stroke volume and the aortic compliance.
Each of the above recited methods has deficiencies which greatly limit either its use and/or functionality for clinical applications, especially during surgery. The Fick principle requires special equipment and careful attention in collecting the required samples and present technology does not allow all of the required patient data to be continuously monitored and analyzed. Non-invasive methods have also demonstrated severe limitations with regard to the size and expense of equipment, the requirement for highly trained personnel and may lead to inaccurate information in patients with cardiac diseases. Finally, the thermodilution technique is not capable of providing real time data on a beat-by-beat basis.
It would be very desirable to provide the clinician with the ability to evaluate cardiac function in certain circumstances, such as with critically ill patients or during surgery, on a continual basis since all other hemodynamic information except cardiac output is currently gathered on a beat-to-beat basis. By obtaining beat-to-beat cardiac output, a hemodynamic assessment of the patient could be performed continuously by the attending staff.
Accordingly, it has been considered desirable to develop a new and improved catheter for measuring cardiac output together with a method for determining the instantaneous volume of blood in a chamber of a heart and a cardiac output monitoring system with which the catheter can be used which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.